In recent times an increased interest is found in powders, especially in applications aiming for the production of thin polyfunctional layers on substrates. Such applications can either aim for uniform layers, or can be aiming for the production of image-wise patterns. The specific technology used to realise such layers can be of different kinds, such as e.g. electrostatic powder spray coating, dipping, etc.. or more sophisticated technologies such as electrophotography for the creation of images starting from electroscopic powder particles, in such case commonly called dry toner particles. In all applications there is a need to go for smaller particles, in order to improve the coating in quality aspects such as smoothness, etc.. but also in order to reduce material cost. However one of the major problems encountered in going for smaller sizes in powders is the increase in specific surface of such powder, resulting in strong increase of cohesion and hence an increased tendency for agglomeration. This results in the presence of larger particles, being said agglomerates, during the coating process, thus jeopardising the benefits of using smaller particles. In most cases this will limit the use of smaller particles, and thus block any further progress of technology towards improved and less expensive thin layers. There is evidence that the actual detailed control of the shape of the particles might also be of relevance, so that not completely spherical particles are of interest, but somewhat potato-like structures.
There has been proposed a lot of methods to tackle said problems, in that spherical or semi-spherical particles are used to improve the resistance towards agglomeration. Another proposed solution to said problem is the design of core shell particles with specific low energy surface so that the cohesion and agglomeration forces are minimised. Although these proposals do indeed diminish the problem of agglomeration of the toner particles, the use of these concepts is strongly limited by the fact that there are only few and rather complex methods to make such rounded and/or surface-designed particles. Some methods for the production of said particles have been proposed. Depending on the method there is room for some control of the potato-shape or not.
In a first method, the particles are produced directly from suspension polymerisation. In some related proposed methods, the particles are produced from binder solutions, being dispersed in a non-compatible liquid, with subsequent evaporation of the solvent used in the dispersed droplets, in order to realise solid particles.
In still another proposed method the particles are prepared in situ by flocculation of pre-particles, e.g. latex particles, either using homo-, hetero, and or thermally assisted and/or flocculant assisted processes. In still another proposed method solid particles are dispersed and thermally heated above their plastification temperature, in most cases corresponding with the Tg of the particles, and upon stirring allowed to take a more spherical shape. Such a process has been described in U.S. Pat. No. 4,345,015 wherein it is disclosed to place irregularly shaped particles in a liquid carrier under stirring and heating them to a temperature at which the resin particles soften until they become spherical.
In all said cases the particles will during some time be present in a liquid, which has to be removed, discarded, the particles to be dried, hereby taking care not to have any drying residue on the surface, to avoid hydrophilicity on the particles, in case electrostatic processes are used in the subsequent application of the powder particles in the coating process, etc.. Also the use of energy to dry, the problems encountered to avoid agglomeration during drying etc. are to be overcome. These problems seriously reduce the usefulness of these approaches.
In a second method the particles are produced by using a hot gas stream. In a first proposed method the non spherical particles are continuously injected and fed in a heated gas stream. The temperature and residence time in the process is chosen in such a way that the particles are allowed to take a more spherical shape. Preferably the temperature is set at a temperature higher than the plastification temperature, i.e. Tg, of the polymers comprised in the particles to be rounded particles. The process can also be carried out as a batch process or as a continuous process. This is known in the art as fluidization. In still another approach a dispersion of the particles is made in a liquid and afterwards spray dried. In that particular case, the evaporation of the dispersant should take place and the amount of energy should be enough to plastisize the particles. In still another method the particles are dissolved and/or dissolved/partially dispersed in a liquid and spray dried in order to remove the solvent/dispersant. During the evaporation of the solvent the particles are dried and formed and spherical particles are formed. The drawback of all the above cited embodiments of the second method is that large amounts of hot gas are necessary and thus also a lot of energy because the spheroidization process can only take place above the plastification temperature. Another drawback is that this process has the tendency to form agglomerates and contamination of the wall of the equipment during the process. To avoid these drawbacks one has to work with small ratios of particles/gas resulting in small throughput of rounded particles and the use of large amounts of heated gas what makes this method from economical viewpoint problematic. Still another serious problem is the fact that a finely divided powder is made in a gas, which might give powder explosion in case oxygen is present. In order to reduce said risks, inert atmosphere is to be used which at such large volumes is extremely expensive. Still another drawback is that at the end the process the particles and heated gas have to be separated. This means special precautions to cool the gas and also actions have to be taken towards dust control in the final `clean` gas stream, resulting again in a cost.
In a third method, as described in U.S. Pat. No. 4,915,987 the particles are shaped by bringing them in a mechanical mixing device wherein by applying mechanical energy and/or thermal energy the particles are rounded. The temperature control of this process is critical. Agglomeration of the particles and contamination of the inner walls of the equipment with plasticized material can easily take place. This necessitates an additional sieving step to get the original particle size. Said temperature control is difficult since the particles are in fluidised state, and this control also necessitates the use of complex and expensive cooling equipment. In order to avoid agglomeration, the particle concentration is kept low thus resulting in a low production speed, since in such devices a batch process is evident. This also rises the cost of particles produced by said method. Also the quality can be degraded by attrition due to the high mechanical impact of the rotor of the mixing device on the particles what means that an additional separation step can be necessary in order to get rid of the fines that are produced during the rounding process. So also in these methods drawbacks are present.
From the above cited methods we can learn that there is still a need for improvement for processes to round particles and for processes allowing for core-shell design on powder particles.